I don't have a dog, of course, but my kitties are leash trained- I prefer a retractable leash for them. When I walk my neighbors dog, Tasha, I also use a retractable for her. I like them because it gives them lots of space to walk and explore, yet keeps them in controled and safe, and you can shorten it when you need to.

When I get a dog, I most likely will use a retractable, and have a nylon as an extra/spare.